It is becoming increasingly desirable to provide on-board diagnostic means for certain components of internal combustion engines, especially components which have a major impact on critical engine performance criteria. This is particularly true in the motor vehicle industry, where high precision in the control of fuel supply to the engine has become essential to various present and planned engine management features designed to meet increasingly strict emissions, performance, drivability, and maintenance objectives. It is now well known how to adjust the fuel flow to the cylinders of an engine to maintain desired fuel/air mixture ratio for meeting engine emission requirements by electronically controlling the actuation timing and duration of the engine's fuel injectors. Electronic fuel injector control may be incorporated into known electronic engine control (EEC) modules performing a variety of engine control functions. In accordance with such known systems, the timing of injector actuation is controlled by the timing of the corresponding actuation signal sent by the control module. The duration of injector actuation, during which fuel is passed through the injector from a fuel rail or like fuel supply means, is controlled by the duration of the actuation signal from the control module, that is, by the pulse width of the signal.
Reliably controlling fuel supply to an engine by controlling fuel injector actuation signal timing and duration (i.e., pulse width) assumes the absence of various possible fuel system problems, such as vapor in the fuel supply line. Thus, especially in support of maintaining the efficacy of electronic engine management devices adapted to control air/fuel ratio by controlling the actuation of fuel injectors, it would be desirable to provide an on-board diagnostic system to periodically test for the presence of fuel line vapor during engine operation. It is a primary object of the present invention to provide such on-board diagnostic system for fuel line vapor. Additional objects and features of various embodiments of the invention will be apparent from the following disclosure.